Electrothermographic image recording process

ABSTRACT

A process of electrothermographic reproduction utilizing an electrothermographic material which undergoes a decrease in electrical resistance as the temperature thereof is increased, the rate of decrease changing to a significantly higher rate at a temperature above room temperature. Preferably a span of not more than 20* C. separates the temperature at which the rate change occurs and the temperature at which the resistance of the material reaches a minimum level. Preferred electrothermographic materials are crystalline polymers having a first order transition point and amorphous polymers having a second order transition point, which transition points are above room temperature.

O United States Patent [151 3,654,461 Conix et al. [4 1 A r. 4 1972 [541 ELECTROTHERMOGRAPHIC IMAGE 2,914,403 11/1959 Sugarman, Jr. ..250/65 RECORDING PROCESS 2,937,943 5/1960 Walkup ..250/65 7 2,647,464 8/1953 Ebert ..250/65 1 Inventors Andre Jan Conix, Hove-Antwerp; Paul 2,798,959 7/1957 Moncrieff-Yeates ..250/65 Maria Cassie", Mensa-Antwerp, both of 2,866,903 12/1958 Berchtold ..250/65 Belswm 2,917,385 12/1959 Byrne ..250/65 [73] Assigneer $31231; Photo-Producten N.V., Mortsel, Primary Examiner wimam F. Lindquist g Attorney-William J. Daniel [22 Filed: Dec. 5, 1960 [2]] Appl. No.: 73,518 [57] ABSTRACT A process of electrothermographic reproduction utilizing an electrothermographic material which undergoes a decrease in [30] Foreign Appli ati Dam electrical resistance as the temperature thereof is increased, Dec. 4, 1959 Great Britain ..41,351/59 the rate of decrease changing to a significantly higher rate at a I temperature above room temperature. Preferably a span of 521 0.5. CI. ..250/65 ZE not than separates temperature which [5]] 603g 13/00 rate change occurs and the temperature at which the re- [58] Field 0' Search l a I 250/495 Zc 65 ZE sistance of the material reaches a minimum level. Preferred I I electrothermographic materials are crystalline polymers hav- 56] Reerences Cited ing a first order transition point and amorphous polymers having a second order transition point, which transition points are UNITED STATES PATENTS above room temperature- 2,859,352 1l/l958 Sugarman, Jr. ..250/65 7 Claims, 4 Drawing Figures f/zcfrarler/zwxaz /c [./mwf I f/ a/af/ie h 4/5/89 flea/m Qadmie Zl/i/j'd/fl/I Bread/q fiearras/zfical/y (flayed 4-? f 5 ////////m \\\\\\\\\\\\\\\\\\\\\\\\\\\\v.

lr raa ad/a awake) an? alyy/m/rade 60 154. fiqyefi 1 Jere/a mar/f 075% Caged ELECTROTHERMOGRAPHIC IMAGE RECORDING PROCESS The present invention relates to the production of images of originals by an electrothermographic process.

The known art of electrophotographic copying as it is usually practiced involves the electric charging in the dark of an electrophotographic plate or sheet comprising a coating of a photoconductive insulating material on a conductive support, in such a way that the coating carries a fairly uniform positive or negative charge. The electrophotographic plate or sheet is then image-wise exposed to an electro-magnetic radiation whereby the irradiated areas of the photoconductive layer are image-wise discharged and an electrophotographic latent image is obtained. This latent image can then be developed by depositing thereon a finely divided electrostatically attractable powder or ink so that a visible image is obtained which, in the case of a powder image, can be fixed by heating, either on the photoconductive layer or on another support, for instance on a sheet of paper to which the powder image is first transferred. Alternatively the latent image can be directly transferred to or induced in another support and then developed.

The various electrophotographic processes known hitherto depend upon the phenomenon that the electric resistivity and/or charge condition of certain substances can be modified by subjecting them to energy. in the form of electromagnetic rays, for instance by irradiation with light of suitable wavelength.

The present invention is based upon the phenomenon, that when certain substances are exposed to the influence of heat energy, their electric resistivity and/or charge condition changes or varies discontinuously with the amount of heat energy supplied as measured, for example, by the temperature of the substance, the term discontinuous being employed to denote the occurrence of a distinct break or deviation in the curve of resistivity vs. heat energy, i.e. temperature, from its. configuration up to that point.

By making use of such a substance (hereinafter described as electrothermographic) having an electrical resistivity which when the substance is heated begins to decrease or to decrease in a discontinuous manner, i.e. begins to decrease or decreases more rapidly when a certain temperature level is reached, the present invention, according to one aspect thereof, provides a new image producing method which comprises producing an electrostatic latent image in or on the said substance either by applying an electrical charge to said substance and image-wise heating the said substance beyond the critical temperature either before or after said electrical charge is applied or by carrying out said heating while the said substance is electrically charged or subjected to charging conditions.

The new method is capable of being performed in a variety of ways corresponding with the various latent image producing techniques proposed in the art of electrophotography using light or other electromagnetic radiation in the exposure step, some of which methods have been briefly referred to above. Development of the latent image, whether or not after transfer, may also be realized by most if not all of the various methods proposed in the electrophotographic art.

The invention also includes electrothermographic materials for performing the new method. Such materials comprise at least one layer or sheet member essentially consisting of an electrothermographic substance or substances with the properties referred to.

As the invention depends firstly on the selection of substances having the required electrothermographic properties, we will first discuss the substances themselves and give examples thereof and will then refer in more detail to certain applications of the substances and to certain embodiments of the new method.

Suitable electrothermographic substances are compounds the electric resistivity of which inside a critical temperature interval decreases more rapidly than outside this interval. The critical temperature interval can coincide with a transition of the first order, such as the melting point, or with a transition of the second order such as the softening points, the glass-transition point or other secondary transition.

electrothermographic material Data so far available indicate that in order to obtain favorable results, the electrothen'nographic compounds preferably show a critical temperature interval situated between 40 and C. and above this critical temperature interval they must have a sufficiently low specific resistivity with respect to the specific resistivity of the electrothermographic layer or sheet at room temperature so that if the layer is first given an overall charge the electrostatic charges in the areas to which heat is adducted can easily flow away. For obtaining sharp prints more particularly by reflex-exposure (i.e. exposure with the positioned between the original and the radiation source) it appears to be important for the substances to have a narrow critical temperature interval, the narrower the better. Indeed, if a layer composed of a substance which, when subjected to heat energy, shows a broad critical temperature interval, or shows a continuous decrease of electric resistivity, is electrostatically charged and irradiated through a transparent original, the electrostatic latent image obtained (if any) cannot be transformed by development into a clear print; moreover, such materials are completely unsuitable for refiectographic exposure. The reason for the insufficient contrast in the obtained print lies in the fact, that in any existing original, the applied radiation will neither be completely absorbed by the characters, nor completely transmitted or reflected by the image background. This means that the actual temperature difference between character and background is very small, and insufficient to allow a satisfactory selectivity in the resulting charge pattern.

In order to establish whether a certain compound is suitable for carrying out the method according to the present invention, a practical photographic test can be executed. lf, however, a large number of compounds have to be examined, this method is rather time-wasting and it is recommended to execute first a pre-selection, e.g., according to the following method, whereby the unsuitable compounds can be excluded. A layer or sheet of the material to be tested is electrostatically charged until the saturation charge is reached, and then pressed for 1 second to a metal plate the temperature of which from one end to the other increases from 50 to 250 C. An example of such a plate is, for instance, the Kofler l-leizbank of the Optische Werke C. Reichert A.G., Vienna. 1f the layer or sheet is immediately thereafter dusted with electroscopic powder, three zones become visible. Below a temperature A the powder is maximally attracted, above a temperature B no powder adheres anymore. Between these zones lies the discharge interval A-B wherein the powder precipitate gradually decreases. Suitable substances show a maximum discharge interval of 20 C. and preferably 10 C.

For carrying out the method according to the present invention, the electrothermographic layer or sheet member wherein one or more of these compounds are present must possess a sufficiently high resistivity at room temperature. For these reasons it is preferable to choose such electrothermographic compounds which at room temperature possess a specific resistivity of at least 10 ohm cm. However, electrothermographic compounds with lower specific resistivity can also be used if a substance with high specific resistivity is added when forming the layers or sheets.

Compounds with electrothermographic properties suitable for carrying out the method according to the present invention can be found in the most widely differing chemical classes, more particularly in the different classes of macromolecular organic compounds, as will appear from the following list which identifies a series of specific electrothermographic macromolecular compounds with which favorable results have been attained. These compounds can be used singly or a combination of two or more compounds may be used.

It has been determined that many of the polymeric substances listed in the U.S. Pat. applications Ser. No. 11,128, filed Feb. 26, 1960, now abandoned, Ser. No. 11,129, filed Feb. 26, 1960, now U.S. Pat. No. 3,155,503 Ser. No. 11,130, filed Feb. 26, 1960, now U.S. Pat. No. 3,131,060, and the complete specification of the British Pat. application Nos. 14,652/60 and 16,188/60, now cognated into British Pat. No.

964,879, are, in general, very suitable for the manufacture of electrothermographic materials for use in carrying out the method of the present invention. Polymeric compounds which have been shown to be especially useful in this respect are as follows (the most interesting of them are marked with an asterisk):

1. Natural polymers and their derivatives such as chlorinated rubber pine resins cellulose derivatives (e.g. cellulose diacetate)* ll. Vinylpolymers e.g. a. Polyacetals* e.g.

formaldehyde-acetals of polyvinyl alcohols such as Formvar 15-95 acetaldehyde-acetals of polyvinylalcohol butyraldehyde-acetals of polyvinyl alcohol such as Vinylite XYHZ b. vinylchloride polymers and copolymers e. g.

polyvinyl chloride chlorinated polyvinylchloride such as Rhenoflex copolymer of vinylchloride/vinylacetate/viny]alcohol such as "Vinylite VAGH copolymer of vinylchloride/vinylacetate/maleic acid such as Hostalit CH Vinylacetate polymers and copolymers e.g.

polyvinylacetate copolymers of vinylchloride and vinylacetate such as Geon Resin 400x65" d. Vinylidenechloride polymers and copolymers e.g.

polyvinylidene chloride copolymer mainly consisting of vinylidene chloride such as Diofan l90-D e. Vinylcarbazole polymers and copolymers, e.g.

polyvinylcarbazole copolymer of Vinylcarbazole and vinylacetate copolymer of Vinylcarbazole and methylmethacrylate f. Acryland methacrylester polymers and copolymers, e.g.

polymethylmethacrylate copolymer of methacrylic acid and methylmethacrylate Plexigum g. Styrene polymers and copolymers h. Polyalkylenes i.e. polymers the chain of which is formed with a succession of units of the same or of different type, corresponding to the formula lmiLl -L 1.

wherein R represents an hydrogen atom, an aliphatic or aromatic radical such as e.g. CH CH(CH CH --Cl-l C H and C H -Cl-l and R represents an hydrogen atom or an aliphatic radical such as e.g. CH e.g. polyethylene, polypropylene, polybutylene.

III. Polycondensates Polycondensates which may be used are those wherein the main chain mainly groupings appear which only to a minor extent contribute to the binding of water-molecules. Such groupings are for instance carboxylic acid ester groups, sulphonic acid ester groups and anhydride and/or ether bonds, Other groupings which possess a more or less hydrophilic character such as for instance carbonamide-, sulphonamide, urethane-, ureaand other bonds may only be present in a low percentage. More in particular the number of these groupings must never be so high that the water-absorption of the polymer on saturation amounts to more than 2 percent. Among others are polycondensates of the polyester type especially suitable for carrying out the method of the present invention. The class of the polyestersis for instance amply described in the book of J Bjorksten, H. Tovey, B. Harker and J. Hemming, Polyesters and their Applications, Reinhold Publishing Corp., 1956. A large number of the polyesters described in this book have been shown to be suitable for carrying out the process of our present invention.

a. Linear polyesters For characterizing the tested polyesters, their intrinsic viscosity [1 is measured in symmetrical tetrachloroethane unless otherwise stated.

1. Among the polyesters with which especially favorable results can be attained are mentioned among others the aliphatic linear polyesters prepared from aliphatic diols and of aliphatic dicarboxylic acids. These polyesters are soluble and can be cast to thin or self-supporting sheets. Most of the polyesters of this class are crystalline and possess melting points (transition temperatures of first order) situated between room temperature and C.

2. Another class of polyesters with which favorable results can be attained are those prepared by starting from aliphatic diols and from aromatic dicarboxylic acids such as for instance: polyhexamethylene-isophthalate, polyester of isophthalic acid and neopentylglycol, polyester of terephthalic acid and ethyleneglycol, copolyester of mixtures of isophthalic acid and terephthalic acid with ethyleneglycol.

. Another class of polyesters with which particularly favorable results are attained are the polyesters and copolyesters prepared by starting from aromatic dihydroxy compounds and from aromatic dicarboxylic acids and/or aromatic disulphonic acids and/or aliphatic unsaturated dicarboxylic acids*. Polyesters of this type as well as their preparation and properties are described among others in Belg. Chem. Ind. 22 (1957), l,457l,462, Ind. and Eng. Chem. 51 (1959), l46-l50 and Kurzmitteilungen, IUPAC Symposium ueber Makromolekulare Chemie, .Wiesbaden, Sektion W B 9, Verlag Chemie 1959; others are the subject of the US. Pat. applications Ser. No. 702,252, filed Dec. 12, 1957, now abandoned Ser. No. 725,498, filed Apr. 1, 1958, now US. Pat. No. 3,028,364, Ser. No. 731,874, filed Apr. 30, 1958, now U.S. Pat. No. 3,216,970, Ser. No. 797,587, filed Mar. 3, 1959, now abandoned, Ser. No. 62,076, filed Oct. 12, 1960 and the British Pat. application No. 14,097/60, now British Pat. No. 988,043, such as for instance: polyester of 4,4-dicarboxydiphenylmethane and 2,2-

(4,4-dihydroxydiphenl)-l 1,-trichloroethane [1 0.86 polyester of 4,4-dicarboxydiphenylether and 2,2-(4,4'-

dihydroxydiphenyl)-propane [1 0.80 polyester of 4,4'-dihydroxydiphenylphenylmethane and benzophenone-4,4-dicarboxylic acid [77] 0.37 polyester of di(p-carboxyphenyl)-sulphone and 2,2-(4,4-

dihydroxydiphenyl)-propane [17] =O.64 polyester of fumaric acid and 1,1 '-(4,4-dihydroxydiphenyl)-cyclohexane [1 0.45 (in 1,2-

dichloroethane) polyester of terephthalic acid and l,l-(4,4'-dihydroxydiphenyl)-2,2,2-trichloroethane [1 0.8 polyester of terephthalic acid and 4,4'-dihydroxydiphenyl-phenylmethane [7;] 0.8 copolyester of a 50:50 mixture of isophthalic acid and terephthalic acid with 2,2-(4,4'-dihydroxy-3,3,5,5-

tetrabromodiphenyl)-propane [7;] =1 l2 copolyester of a 50:50 mixture of terephthalic acid and isophthalic acid with 2,2-(4,4'-dihydroxy-3,3-5,5-

tetrachlorodiphenyl)-propane ['n] 1.50 polyester of isophthalic acid and 4,4'-dihydroxydiphenyldiphenylmethane [1;] 0.12 polyester of isophthalic acid and 4,4'-dihydroxydiphenylmethylphenylmethane [1;] 0.74 polyester of isophthalic acid and 4,4-dihydroxydiphenylmethyl-isopropylmethane [1 =0.8

polyester of isophthalic acid and 2,2-(4,4'-dihydroxy-3,3-

dimethyldiphenyl)-propane [1 1.1

polyester of diphenylether-4,4'-disulphonic acid and 2,2-

(4,4'-dihydroxydiphenyl)-propane [1 1.4

polyester of diphenylether-4,4'-disulphonic acid and 4,4-

dihydroxydiphenyl-phenylmethane [1;]= 0.67

polyester of diphenyl-4,4'-disulphonic acid and 4,4- dihydroxydiphenylmethylphenylmethane [1 1.04

polyester of diphenyl-4,4'-disu1phonic acid and 4,4-

dihydroxydiphenylphenylmethane {17]= 1.5

polyester of diphenyl-4,4'-disulphonic acid and 2,2-(4,4'-

dihydroxydiphenyl)-butane [1 1.65

polyester of diphenylmethane-4,4'-disulphonic acid and l ,1-(4,4dihydroxydiphenyl)-cyclohexane [1;] 0.85

polyester of diphenyl-4,4'-disulphonic acid and 3,3-(4,4'-

dihydroxydiphenyl)-pentane [1;] l

polyester of diphenyl-4,4-disulphonic acid and 2,2-(4,4- dihydroxy-3,3'-dimethyl) -diphenylpropane [1;] 1.1

polyester of diphenylether-4,4'-disulphonic acid and 3,3-

(4,4'-dihydroxydiphenyl)-pentane [1 0.67

polyester of diphenyl-4,4-disulphonic acid and 2,2-(4,4'-

dihydroxydiphenyl)-4-methylpentane [17] 0.5

polyester of diphenyl-4,4'-disulphonic acid and l,l-(4,4- dihydroxy-3,3'-dimethyldiphenyl)-cyclohexane [17] 1.4

polyester of diphenylsulphide-4,4'-disulphonic acid and 2,2-(4,4-dihydroxydiphenyl)-propane ['17] 1.04

polyester of benzene-1,3-disulphonic acid and 2,2-(4,4-

dihydroxydiphenyl)-propane [1 =0.8

polyester of 1-chlorobenzene-3,S-disulphonic acid and 2,2-(4,4'-dihydroxydiphenyl)-propane polyester of furane-2,5-dicarboxylic acid and.2,2-(4,4'-

dihydroxydiphenyl)-propane [17] 0.50

copolyester mixtures of diphenylether-4,4'-dicarboxylic acid and diphenyl-4,4'-disulphonic acid with 2,2-(4,4'- dihydroxydiphenyl)-propane [1 1.62

copolyester of mixtures of terephthalic acid and diphenyl- 4,4-disulphonic acid with 2,2-(4,4'-dihydroxydiphenyl)-propane [1 1.65

copolyester of mixtures of diphenyl-4,4-disulphonic acid and terephthalic acid with 2,2-(4,4-dihydroxy-3,3'- dimethyl-diphenyl)-propane [1 0:76.

4. Still another class of polyesters with which favorable results are attained are the polycarbonates, i.e. polyesters wherein the ester function appearing in the main chain is that derived from carbonic acid; they are built up from structural units of the formula wherein R represents the residue of a dihydroxy compound.

Among the polycarbonates are to be mentioned the aliphatic polycarbonates derived from aliphatic dihydroxy compounds and more particularly the aromatic polycarbonates derived from diphenols. The latter are extensively described in Angew. Chem. 68 (1956) pp. 633-640 and in Ind. and Eng. Chem. 51 (1959) pp. 157-160. Also the cycloaliphatic polycarbonates, which can be prepared from cycloaliphatic diols and dialkyl or diaryl carbonates such as for instance may be mentioned.

5. Polyphosphonates such as e.g. the polyester derived from phenylphosphonic acid and 2,2-(4,4'-dihydroxy-3,3',5,5' -tetrachlorodiphenyl)-propane.

b. Non-linear polyesters It is evident that in the preparation of the polyesters, instead of difunctional starting products also polyfunctional starting products can be used, for instance tetrafunctional glycols such as pentaerythtitol. In this case, however, no linear soluble polyesters but branched or cross-linked polyesters are obtained.

Electrothermographic material according to the invention may comprise an electrothermographic compound or compounds in the form of a layer upon a support or in the form of a self-supporting sheet.

The thickness of the electrothermographic layer or sheet is not critical, but is open to choice within a wide range according to requirements in each individual case. Good results have been attained with electrothermographic layers or sheets of a thickness between lp. and 20 4.; the preferred thicknesses are from Sp, to 10p. Indeed, layers or sheets which are too thin have insufficient insulating power and layers or sheets which are too thick require extensive exposure times.

The electrothermographic layers or sheets may be composed of one electrothermographic compound or of a blend of two or more electrothermographic compounds with similar or different photo-electrical, thermal, mechanical or other physical properties, or of a mixture of an electrothermographic compound with monomeric or polymeric compounds for imparting the desired properties to either the layer or sheet itself and/or to a coating composition if the layer or sheet is formed from such acomposition. Plasticizers such as dibutylphthalate, dimethylphthalate, dimethylglycolphthalate, tricresylphosphate, triphenylphosphate, monocresyldiphenylphosphate, etc., may be added to the electrothermographic polymers in amounts of 10 to 30 percent of the polymer weight.

Other additives, well known in the art of applied coatings, may be used in the formation of electrothermographic layers or sheets, e.g., pigments and agents controlling viscosity, free flow, aging and thermal stability, oxidation and gloss. In selecting such additives, preference is given to those which do not markedly impair the dark-resistivity of the material.

For carrying out the new method according to the present invention it is also possible to use an electrothermographic material which contains several superposed layers and/or sheets formed from electrothermographic compounds and either differing from each other or not.

In carrying out the method according to the present invention a layer more conductive than the electrothermographic layer is united with or at least held in intimate surface contact with a backing in the form of an electrically-conductive plate or sheet member, or in the form of an insulating plate or sheet with an electrically-conductive layer in contact with the electrothermographic layer. An electrically-conductive member or layer is a member or layer the specific resistivity of which is smaller than that of the electrothermographic layer, i.e. in general smaller than 10 ohm cm. conductive materials the specific resistivity of which is smaller than 10 ohm cm. are preferably used.

Suitable insulating sheets are for instance glass plates; these plates must be coated with a conductive layer, for instance with a transparent layer of silver, gold or stannous oxide deposited thereon by vacuum evaporation.

Suitable insulating sheets are for instance films of synthetic macromolecular substances with high specific resistivity such as for instance the polysulphonates described and claimed in U.S. Pat. applications Ser. No. 797,587, filed Mar. 6, 1959 now abandoned, and Ser. No. 62076, filed Oct. 12, 1960, now abandoned, polyesters such as those described in U.S. Pat. applications Ser. No. 702,252, filed Dec. 12, 1957, now abandoned, Ser. No. 725,498, filed Apr. 1, 1958 now U.S. Pat. No. 3,028,970, and Ser. No. 731,874, filed Apr. 30, 1958, polystyrene, polyethylene, cellulose esters etc. or sheets of paper with high specific resistivity. These insulating sheets must be provided with a conductive coating, e.g., with a thin metal sheet, with a layer comprising a metal powder dispersed in a binding agent (which will be used in as small a proportion as possible), or with a thin hydrophilic layer comprising an hygroscopic and/or antistatic compound and an hydrophilic binding agent; suitable hygroscopic and/or antistatic compounds are for instance glycerine, glycol, polyethylene glycols, calcium chloride, sodium acetate, condensation products of maleic acid and polyethylene glycols, citric acid amides, hydroxypropylsucrose-monolaurate, quaternary ammonium compounds such as Antistatin LF (trade name of Badische Anilinund Soda Fabrik A.G., Ludwigshafen/Rh), Arquad l6 and Ethoquad 18/25 (trade names of Armour and Company, Chicago, Ill.), amine salts of lyophilic alkyl phosphates such as e.g. Zelec NK (trade name of E. l. du Pont de Nemours and Co., Wilmington, Del.), lyophilic dialkyl polyoxyalkylene phosphates such as e.g. Victawet 12 (trade name of Victor Chemical Works, Chicago, Ill.) and polyoxyalkylene amides such as Ethomid PIT/60 (trade name of Armour and Co., Chicago, 111.); suitable hydrophilic binding agents are e.g., gelatin, glue, polyvinyl alcohol, methylcellulose, carboxymethylcellulose, cellulose sulphate, cellulose hydrogen phthalate, cellulose acetate sulphate, hydroxyethyo cellulose, polyacrylic acid or colloidal silica; for obtaining a good adhesion of the hydrophilic layer to the hydrophobic polymeric sheet, the polymeric sheet can be provided with a suitable subbing layer such as for instance one of the subbing layers disclosed in the British Pat. No. 782,165 (for polyester films), the British applications No. 819,592 and No. 41732/58 (now British Pat. No. 563,475 and the Belgian Pat. No. 587,302, for films of polystyrene and substituted polystyrene), the British Pat. application No. 22508/58, now British Pat. No. 910,308, and the Belgian Pat. No. 569,129 (for polyalkylene films), the British Pat. No. 799,184 (for cellulose ester films) and the Belgian Pat. No. 573,005 (for hydrophilic layers with polyvinyl alcohol as binding agent).

Suitable conductive plates are for instance plates of metals such as aluminum, zinc, copper, tin, iron or lead.

Suitable conductive sheets are for instance films of polymeric substances with low specific resistivity such as for instance polyamide films or paper sheets with low specific resistivity. Good results can be attained when using paper sheets containing hygroscopic and/or antistatic substances such as those described hereinbefore. These hygroscopic and/or antistatic substances are preferably incorporated into the paper sheets during the paper manufacturing process either by adding them to the paper pulp or by an after-treatment, before or after calendering the paper sheets. The substances involved can also be incorporated into the paper sheet by applying to the raw paper stock a composition containing the hygroscopic and/or antistatic substances and an hydrophilic binding agent such as described hereinbefore.

Especially good results can be attained when using paper sheets with low infrared absorption, such as sheets of Pergamin paper, of parchment paper or of sulphite cellulose paper. For attaining optimal results it has been found that paper sheets with a weight of 30-60 g./sq. m. are most suitable.

Good results have also been attained when using as backing paper sheets provided with a coating of a polymeric substance in order to obtain a smooth surface and to prevent the organic liquid wherein the electrothermographic substance or substances is (are) dissolved from penetrating within the paper sheet. This coating, however, must not prevent the carryingoff of the electrons from the irradiated areas during the irradiation. Suitable coatings are for instance layers with a thickness of 2p. to and composed of one or more of the following polymeric substances polyvinyl acetate, vinyl acetate copolymers, polyacrylic acidand polymethacrylic acid esters, silicon resins, polystyrene, styrene copolymers, polyvinyl acetal, polyvinyl formal, polyvinyl butyral, polyethylene, alkyd resins, glycerophthalate resins, modified colophonium resins such as penterythritesters, ethylene glycol esters, diethylene glycol esters, glyceryl esters, ester gum, formaldehyde resins, phenolmaleic resins, coumarone indene resins, microcrystalline waxes, cellulose derivatives such as ethyl cellulose, ethyl cellulose stearate, cellulose diacetate and triacetate, cellulose acetobutyrate and finally natural resins such as dammar resins, mastic gum, elemi resins, manilla gum and sandarac. If desired plasticizers can be added such as for instance dibutylphthalate, dimethylphthalate, dimethylglycol phthalate, tricresylphosphate, triphenylphosphate and monocresyldiphenylphosphate, etc. Especially good results were attained when using polysulphonates such as those described in the US. Pat. applications Ser. No. 797,587, filed Mar. 6, 1959, now abandoned and Ser. No. 62,076, filed Oct. 12, 1960, now abandoned, and the polyesters disclosed in US. Pat. applications Ser. No. 702,252, filed Dec. 12,1957, now abandoned, Ser. No. 725,498, filed Apr. 1, 1958, now US Pat. No. 3,028,364, Ser. No. 731,874, filed Apr. 30, 1958, now US. Pat. No. 3,216,970, and British Pat. application No. 35430/57, now cognated into British Pat. No. 901,605.

Besides the usual kinds of paper synthetic types of paper can likewise be used such as those prepared from polyesters, e.g. Dacron and Terlenka fibers (trade names for polyester fibers from terephthalic acid and glycols, manufactured by E. l. du Pont de Nemours and Co. lnc.,Wilmington, DeL, resp. Algemene Kunstzijde Unie N,V., Arnhem, Netherlands) from polyamide fibers such as Enkalon fibers (trade name of Algemene Kunstzijde Unie N.V., Arnhem, Netherlands) or Nylon fibers, or from polyacrylonitrile fibers such as Orlon fibers (trade name of E. I. du Pont de Nemours and Co. lnc., Wilmington, Del.). Before coating the electrothermographic layers on such paper sheets, the latter are preferably impregnated with a substance enhancing their conductivity for instance with polycaprolactam, the polyester of 2,5-di(p-oxyphenyl, 1,3,4-oxadiazole with l-chloro-3-aminobenzene-4,6- disulphocloride, a copolyamide of hexamethylenediamine, caprolactarn, adipic acid and sebacic acid, N-methylenepolyhexamethylene adipamide, or with polyamides such as Ultramid C1 (trade name for a polyamide manufactured by Badische Anilin- & Soda Fabrik A.G., Ludwigshafen/Rh).

When employing an electrothermographic compound or compounds worked up into a self-supporting sheet, a conductive layer may be used in the form of a backing consisting of a thin metal sheet which may be anchored to the electrothermographic sheet by means of a suitable subbing layer, of a thin metal layer formed by vacuum evaporation, or of a layer comprising a finely divided metal power and a binding agent, the latter being used in an amount as small as possible. Suitable conductive backings for electrothermographic polymeric sheets can also be found for instance among the coatings described as subbing layers in the British Pat. No. 782,165 (for polyester films) and the Belgian Pat. No. 569,129, and the British Pat. application No. 22508/58, now British Pat. No. 910,308 (for polyalkylene films).

By way of example, the new image-forming method may be performed as follows: The electrothermographic layer or sheet is electrostatically charged with a positive or negative charge according to one of the methods known to those skilled in the art of electrophotography, for instance by friction with a smooth material or with a material having a high electric resistivity such as for instance a cylinder coated with polystyrene, by corona discharge, by contact charging or by discharging a condenser. The charging can be effected at the front side or, if the conductive backing is not a metal layer, plate or sheet, at the rear side of the electrothermographic material. A positive charge applied through the rear side of the material gives substantially the same result as a negative charge applied through the front side.

The charged electrothermographic material is then brought either with its front or rear side into close contact with an original having a pattern consisting of portions highly absorptive of radiant energy, and is exposed to an intense radiation. Originals meeting these requirements are, e.g., text or drawings formed with graphite-containing ink such as black printing ink, typewriter ink and India ink, with carbon paper or with pencil. It is also possible to copy colored originals made with colored inks highly absorptive of radiant energy such as e.g. the inks Blau Farbe Nr. AB 9136, Violett Farbe Nr. AB 9135, Gelb Farbe Nr. AB 9138, Griin Farbe Nr. AB

9137, Rot Farbe Nr. AB 8908 and Orange Farbe Nr. AB 9134 marketed by Chr. I-Iostmann-Steinbergsche Farbenfabriken, Celle. The image areas of the original absorb the radiation and convert it into heat. This heat pattern increases the temperature of the corresponding contacting areas of the charged electrothermographic material to beyond the critical temperature interval so that the charges flow away and a latent image is formed. In principle any intensive radiation sources can be used. Particularly good results are attained with infrared lamps placed at a distance of 0.2 to 2 cm., preferably at 0.7 to 1.4 cm. from the printing material. The radiation source can be fixed with respect to the printing material; a more uniform exposure, however, is obtained by moving the lamp gradually over the printing material or by guiding the printing material gradually past the lamp. Favorable motion speeds are from 0.1 to 10 cm./sec., preferably, however, between 1 to 5 cm./sec. The best results are attained with very intensive and very brief irradiations. A close contact between the electrothermographic material and the original is important and is already promoted by the electrostatic charge on the material. The contact can still be improved by pressing both materials into contact with each other on a slightly bent surface with a sheet or screen having a low absorption coefiicient for infrared. Good results are attained with sheets made of Teflon and of Hostafion, respective trade marks of E. I. du Pont de Nemours & Co., (Inc.), Wilmington, Del. for polytetrafluorethylene and of Farbwerke I-Ioechst A. G. Frankfort on Main Hochst, West Germany, for polychlorotrifluorethylene, further with silk or glass fiber screens.

Preferably, the exposure is carried out in a refiectographic way, i.e. with the electrothermographic material positioned between the light source and the original; in this way, by placing the electrothermographic material with its rear side against the original, transfer of the latentor powder image is no longer necessary The latent image resulting from the foregoing steps can then be developed, e.g., by dusting the electrothermographic material with a negatively or positively charged powder as the case may be in the form of an aerosol or in the form of a suspension in an insulating liquid, or by spraying thereon a finely divided electrostatically charged liquid, according to one of the methods known in the art of electrophotography or according to the method disclosed in the US. Pat. application Ser. No. 741,017, filed June 10, 1958, now British Pat. No.-

9l0,308, or in the U.S. Pat. application Sr. No. 856,357, filed Dec. 1, 1959, now abandoned.

By suitable choice of the sign of the charge to be applied and of the developing powder, a negative or positive print can be obtained at will from any original. If the electrothermographic material and the developing powder carry a charge of the same sign, the powder only adheres to the discharged areas and a print with the same image value as the original is obtained (positive/positive). If the sign of the charge of the material and the developing powder are different, the image values are reversed (negative/positive).

The visible image thus obtained can then, if necessary, be fixed according to one of the methods known in the art of electrophotography, for instance by heating, or it can be transferred onto another support and fixed thereon.

An understanding of the aforegoing description may be facilitated by reference to the annexed drawing which shows in step-by-step diagrammatic fashion, one embodiment of the method of the present invention using the preferred reflectographic technique of exposure, the materials and the steps employed in that method being appropriately captioned.

There are of course many other ways of applying the present invention in the production of visible images. Thus, for example, it is possible to transfer the formed latent image onto another support or to a plurality of further supports (multi-copies) according to one of the methods known in electrophotography and to develop the latent-image(s) on such other support(s).

It is an important advantage of the method according to the present invention over the known electrographic processes that many of the electrothermographic materials, even after charging, are generally insensitive to daylight or artificial light of mere (ordinary) intensities. Obviously this facilitates the application of the invention to a considerable extent since a dark room or a room with specially adapted illumination is no longer required.

In comparison with the known thermic systems the present invention offers several advantages which cannot be undervalued, such as for instance an unlimited stability of the sensitive printing material before as well as after the treatment. This is not the case in the known thermic systems.

A further decisive advantage in comparison with the known thermic systems is the possibility to develop prints in different colors. It is even possible to make multi-colored prints from a multi-colored original by exposing the sensitive material consecutively through two or more monochrome part originals, to develop each of the formed latent images with a suitable dye powder and to fix the formed color image.

One of the greatest advantages, however, of the method according to the present invention lies in the fact that by application of reflex exposure high contrast can be obtained, which is generally not the case in the known electrophotographic processes and transfer of the latentor powder image can be avoided by placing, during the exposure, the electrothermographic material with the electrothermographic layer or sheet turned to the radiation source, between the radiation source and the original.

The following examples illustrate the present invention without limiting, however, the scope thereof.

EXAMPLE 1 Pergamin paper of 40 g./sq. m. is dip-coated on its front side with the following composition:

polyester of benzene-1,3-disulphonic acid and 2,2-(4,4-dihydroxydiphenyl)-propane 5 g.

methylene chloride 50 cm.

tetrahydrofurane 50 cm.

The layer is then dried with warm air. Its thickness amounts to 7,u. The material is positively charged and placed with the rear side onto an original printed on both sides. A silk gauze No. is braced over the electrothermographic material and the whole is irradiated for 10 sec. with an Osram Theratherm lamp of 500 watts, placed at a distance of 1 cm. The printing material is now removed and dusted with a mixture of 100 parts of glass beads and 5 parts of Toner P 648 marketed by General Photo Products Co. Inc., Chatham, NJ. After fixing the powder image by heat, a strong positive print of the original is obtained.

EXAMPLE 2 Opaline paper of 52 g./sq. m. is coated on its front side with a polyethylene layer of 25 g./sq. m.

a. This material is positively charged and placed with its front side against an original printed on both sides. A braced I-Iostaflon sheet ensures an intimate contact between the original and the electrothermographic material. A Philips lamp of the type l3352/E I3 is moved at a distance of 0.3 cm. over the rear side of the electrothermographic material with a speed of l cm./sec. The printing material is removed and dusted as in example I. A sheet or writing paper is placed onto the powder image and a positive corona charge is applied through the rear side of said writing paper. A strong clearly legible positive print of the original is obtained.

b. This material is charged and exposed as described sub a.,the electrothermographic material however being placed EXAMPLE 3 A solution comprising 12 percent of Vinylite VAGH (a trade mark for a terpolymer of vinylchloride, vinylacetate and vinylalcohol manufactured by Union Carbide and Carbon, New York, NY.) 0.5 percent of castor oil and 87.5 percent of acetone is dipcoated onto parchment paper of 35 g./sq. m. The dried layer has a thickness of 8p.

a. The material is then charged with a negative corona and placed with the front side onto a negative line original (white lines on black ground). A silk screen braced over the material ensures the contact desired. The whole is now moved with the electrothermographic material turned to the light source at a distance of 0.7 cm. along a General Electric lamp 1000 T 3 fastened in a reflector and with a speed of 3 cm./sec. The printing material is removed and brought into contact with a sheet of paper 90 g./sq. m. previously coated with a same polymer layer but having a thickness of 414.. A negative corona charge through the rear side of the latter transfer paper brings the latent electrostatic image to the transfer paper where it is converted by dusting into a positive print (black lines on white background). A suitable powder composition is for instance a mixture of 20 parts of iron powder and 1 part of a molten and pulverized mixture of 10 parts of colophonium, 1 part of carbon black and 1 part of nigrosine. After renewed charging, the printing material is ready for a similar process.

b. The material obtained is charged with a positive corona and placed with the rear side onto a positive line original (black lines on white ground) and is exposed as described sub a. The electrostatic image obtained is developed with the powder composition described sub a. and the resulting positive powder image is fixed by heat.

EXAMPLE 4 The front side of a 40 g./sq. m. Pergamin paper is coated with the following composition:

Formvar 15-95 E (a trade mark for polyvinyl formal resin manufactured by Shawinigan Resins Corporation, Springfield, Mass, g.

acetone 70 cm.

methylene chloride 30 cm.

The layer is dried whereafter its thickness amounts to 7,u.. The rear side of said Pergamin paper is coated with the following composition:

Formvur [-95 E 5 g. titanium dioxide 3 g. acetone 100 cm.

After drying the thickness of this layer amounts to 4p.. An electrostatic charge of +600 volts is applied on the front side and the charged material is placed with its rear side onto a page of a book, printed on both sides. The whole is strongly kept together by a braced sheet of Teflon and then moved with a speed of 2 cm./sec. under a General Electric quartz infrared lamp 1350 T 3 fastened at a distance of 0.5 cm. After removing the printed material, the latent electrostatic image is made visible by dusting it with the powder mixture as described in example 1. After fixing by heat a strong positive print on white background is obtained.

EXAMPLE 5 A sheet of cellulose diacetate (with a degree of substitution of 2.5) of a thickness of 30p. is provided on the rear side by 12 vacuum evaporation with an aluminum layer of a thickness of 2 a. The sheet is charged with a positive corona and exposed through a diapositive, the cellulose diacetate being in close contact with the diapositive. The light source is a General Electric 1000 T 3 lamp which at a distance of 0.6 cm. is moved over the materials with a speed of l cm./sec. After development with the powder mixture as described in example 3, a strong positive print is obtained.

b. The sheet is charged with a positive corona, placed with its rear side onto an original (e.g. a typewritten letter) and held in intimate contact therewith by means of a silk gauze, and exposed as described sub a., the electrothermographic material however, being turned to the light source which is moved with a speed of 1.7 cm./sec. After development and fixing as described in example 1, a positive print is obtained.

EXAMPLE 6 Pergamin paper of 35 g./sq. m. is coated with a solution of the following composition:

polyester of diphenylmethane-4,4'-

disulphonic acid and l,l-(4,4-dihydroxydiphenyl)-cyclohexane 10 g.

methylene chloride cm.

tetrahydrofurane 20 cm.

The layer is dried whereafter the thickness amounts to 7p.. The material is positively charged and exposed as described in example 4. The printing material is now immersed for 1 sec. in a dispersion of the following composition:

hexane 100 cm.

Bellacolorblau B 15 (a trade mark for a blue printing ink manufactured byliinecke- Schneemann, K.G. Druckfarbenfabrik,

Hannover) 100 mg.

A strong positive print in blue color is obtained. By using other inks, development in all colors desired can be carried out. For obtaining prints in two or more colors the following method is used. An electrothermographic sheet is charged, exposed and developed with, e. g., a cyan-colored ink. After drying, the material is charged again and exposed to the same original and developed with a yellow ink. After a third similar treatment, a development with a magenta ink is carried out.

EXAMPLE 7 Pergamin paper of 40 g./sq. m. is consecutively coated with the following composition:

on the rear side Hostalit CAM (trade mark for a terpolymer ofvinylchloride, vinyl acetate and maleic anhydride,

manufactured by Farbwerke Hoechst A.G. Frankfurt (Main)) 10 g. titanium dioxide 5 g. acetone 100 cm.

The layer is dried whereafter the thickness amounts from Sp. to 6 on the front side Hostalit CAM l0 g.

acetone 100 cm.

and after drying with the following composition:

Vinylite XYHL (a trade mark for a polyvinylbutyral resin manufactured 5 g.

by Union Carbide and Carbon, New

York, NY.)

ethanol em.

butylacetate 10 cm.

The total thickness of the layers on the front side amounts to lp.. By application of the process as described in example 4, a very strong print on a clear background is obtained on this material.

EXAMPLE 8 This example is concerned with an electrothermographic material wherein a transition temperature of the first order is used, and wherein a material changes from the crystalline state into the amorphous state. Pergamin paper of 40 g./sq. m. is coated with a solution of polyethylene sebacate in methylene chloride. The thickness of the layer amounts to 9a.

a. After charging with a positive corona, this material is brought into contact by its rear side with a paper showing a printed text and for some seconds exposed to an intensive light source whereby radiation is absorbed by the printed characters and converted to heat. After dusting with a positively charged powder, a good visible image immediately appears, which image is fixed in the usual manner. The complete treatment can take place in full day-light. The explanation of the phenomenon is probably to be found in the fact that polyethylene sebacate changes at 83 C. into the amorphous state, as can be readily observed in a microscope under polarized light by the disappearing of the spherulites. This change is accompanied by an increase in volume and a reduction of the electric resistivity. On all image areas of the original wherein heat is formed by the absorption of radiation, this critical temperature is temporarily exceeded with the consequence that the charge present is led off. On development, only these uncharged areas take up the positively charged powder.

b. The electrothermographic material is exposed as described sub a and l to 2 seconds thereafter, after exposure, charged with a positive corona. Development and fixing is carried out as described sub a.

We claim:

1. A process of forming a latent electrostatic image by means of heat energy which comprises:

1. providing an electrothermographic element comprising an electrically-conductive backing and a layer of an electrothermographic material, the electrical resistivity of which is a discontinuous function of temperature with a negative coefficient characterized by a point of inflection at a temperature above room temperature;

2. applying an electrical charge to said'layer of electrothermographic material;

3. bringing said electrothermographic element into close contact with an original to be copied bearing indicia of an infrared absorptive material; and

4. exposing said electrothermographic element while in close contact with said original to infrared radiation of such intensity and for a sufficient time as to heat the portions of said layer of electrothermographic material corresponding to said indicia to a temperature above that of said inflexion point by virtue of the absorption of said radiation by the indicia material, while the remaining portions of said layer are below said temperature, whereby the portions of said layer corresponding to the indicia of said original are free of electrical charge while the remainder of said layer bears such a charge producing a latent electrostatic image of said original in said layer.

2. A process according to claim 1 wherein said inflection point occurs in the range of about 40l 60 C.

3. A process according to claim 1 wherein the resistivity of said layer of electrothermographic material decreases from said inflection point to substantially its minimum value within not more than about C. above the inflection point temperature and the time and intensity of said exposure to infrared radiation are sufficient to heat the portions of said layer of electrothermographic material corresponding to said indicia at least 20 C. above said inflexion temperature.

4. A process as in claim 1 wherein said electrothermographic element is disposed between said original and the source of said infrared radiation while being exposed to said radiation.

5. A process of forming a latent electrostatic image by means of heat energy which comprises:

1. providing an electrothermographic element comprising an electrically-conductive backing and a layer of electrothermographic material, said electrothermographic material being selected from the group consisting of crystal line polymers having a first order transition point and amorphous polymers having a second order transition point, each of said transition points being at a temperature above room temperature;

2. applying an electrical charge to said electrothermographic layer;

3. bringing said electrothermographic element into close contact with an original to be copied bearing indicia of an infrared absorptive material; and

4. exposing said electrothermographic element while in close contact with said original to infrared radiation of such intensity and for suff cient time as to elevate the temperature of the polymer layer in those regions corresponding to the indicia to a temperature above the transition point temperature of the polymer while the remaining portions of said layer are below said temperature, whereby the portions of said layer corresponding to the indicia of said original are free of electrical charge while the remainder of said layer bears such a charge producing a latent electrostatic image of said original in said layer.

6. A process according to claim 5 wherein said polymer had a specific resistivity of at least 10 ohm cm. at room temperature and said layer is a form of at least substantially self-supporting sheet.

7. A process of forming a latent electrostatic image by means of heat energy which comprises:

1. providing an electrothermograhic element comprising an electrically-conductive backing and a layer of an electrothermographic material, the electrical resistivity of which is a discontinuous function of temperature with a negative coefficient characterized by a point of inflection at a temperature above room temperature;

2. applying an electrical charge to said layer of electrothermographic material;

3. placing the electrically-conductive backing of said electrothermographic element in close contact with an original bearing indicia of an infrared absorptive material; and

4. impinging infrared radiation upon the electrothermographic layer of said element while the backing thereof is in close contact with said original, said infrared radiation being of such intensity and continued for a sufficient time as to heat the portions of said layer corresponding to said indicia to a temperature above said inflection point temperature by virtue of the absorption of said radiation by the indicia material, while the remaining portions of said layer are below said temperature, whereby the portions of said layer corresponding to the indicia of said original are free of electrical charge while the remainder of said layer bears such a charge producing a latent electrostatic image of said original in said layer. 

2. A process according to claim 1 wherein said inflection point occurs in the range of about 40*-160* C.
 2. applying an electrical charge to said electrothermographic layer;
 2. applying an electrical charge to said layer of electrothermographic material;
 2. applying an electrical charge to said layer of electrothermographic material;
 3. placing the electrically-conductive backing of said electrothermographic element in close contact with an original bearing indicia of an infrared absorptive material; and
 3. bringing said electrothermographic element into close contact with an original to be copied bearing indicia of an infrared absorptive material; and
 3. bringing said electrothermographic element into close contact with an original to be copied bearing indicia of an infrared absorptive material; and
 3. A process according to claim 1 wherein the resistivity of said layer of electrothermographic material decreases from said inflection point to substantially its minimum value within not more than about 20* C. above the inflection point temperature and the time and intensity of said exposure to infrared radiation are sufficient to heat the portions of said layer of electrothermographic material corresponding to said indicia at least 20* C. above said inflexion temperature.
 4. A process as in claim 1 wherein said electrothermographic element is disposed between said original and the source of said infrared radiation while being exposed to said radiation.
 4. exposing said electrothermographic element while in close contact with said original to infrared radiation of such intensity and for sufficient time as to elevate the temperature of the polymer layer in those regions corresponding to the indicia to a temperature above the transition point temperature of the polymer while the remaining portions of said layer are below said temperature, whereby the portions of said layer corresponding to the indicia of said original are Free of electrical charge while the remainder of said layer bears such a charge producing a latent electrostatic image of said original in said layer.
 4. exposing said electrothermographic element while in close contact with said original to infrared radiation of such intensity and for a sufficient time as to heat the portions of said layer of electrothermographic material corresponding to said indicia to a temperature above that of said inflexion point by virtue of the absorption of said radiation by the indicia material, while the remaining portions of said layer are below said temperature, whereby the portions of said layer corresponding to the indicia of said original are free of electrical charge while the remainder of said layer bears such a charge producing a latent electrostatic image of said original in said layer.
 4. impinging infrared radiation upon the electrothermographic layer of said element while the backing thereof is in close contact with said original, said infrared radiation being of such intensity and continued for a sufficient time as to heat the portions of said layer corresponding to said indicia to a temperature above said inflection point temperature by virtue of the absorption of said radiation by the indicia material, while the remaining portions of said layer are below said temperature, whereby the portions of said layer corresponding to the indicia of said original are free of electrical charge while the remainder of said layer bears such a charge producing a latent electrostatic image of said original in said layer.
 5. A process of forming a latent electrostatic image by means of heat energy which comprises:
 6. A process according to claim 5 wherein said polymer had a specific resistivity of at least 109 ohm cm. at room temperature and said layer is a form of at least substantially self-supporting sheet.
 7. A process of forming a latent electrostatic image by means of heat energy which comprises: 